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ARINC 429

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ARINC 429 [1] is the technical standard for the predominant avionics data bus used on most higher-end commercial and transport aircraft[2]. It defines the physical and electrical interfaces of a two-wire data bus and a data protocol to support an aircraft's avionics local area network.

Technical Description

Messages

ARINC 429 is application-specific standard for aircraft avionics. It uses a full duplex data bus (Tx and Rx are on separate ports) known as the Mark 33 Digital Information Transfer System (DITS). The physical connection wires are twisted pairs carrying balanced differential signalling. Data words are 32 bits in length and most messages consist of a single data word. Messages are transmitted at either 12.5 or 100 kbit/s[3] to other system elements that are monitoring the bus messages. The transmitter constantly transmits either 32-bit data words or the NULL state. A single wire pair is limited to one transmitter and no more than 20 receivers. The protocol allows for self-clocking at the receiver end, thus eliminating the need to transmit clocking data that existed in previous (6 wire) protocols like ARINC-568.

Word Format

Each ARINC word is a 32-bit value that contains five fields:

  • Bit 32 is the parity bit, and is used to verify that the word was not damaged or garbled during transmission.
  • Bits 30 to 31 is the Sign/Status Matrix, or SSM, and often indicates whether the data in the word is valid.
    • OP (Operational) - Indicates the data in this word is considered to be correct data.
    • TEST - Indicates that the data is being provided by a test source.
    • FAIL - Indicates a hardware failure which causes the data to be missing.
    • NCD (No Computed Data) - Indicates that the data is missing or inaccurate for some reason other than hardware failure. For example, autopilot commands will show as NCD when the autopilot is not turned on.
      • The SSM can also indicate the Sign (+/-) of the data or some information related to it like an orientation (North/South/East/West).
  • Bits 11 to 29 contain the data. Bit-field, Binary Coded Decimal (BCD), and two's complement binary encoding (BNR) are common ARINC 429 data formats. Data formats can also be mixed.
  • Bits 9 and 10 are Source/Destination Identifiers (SDI) and indicate for which receiver the data is intended or more frequently which subsystem transmitted the data.
  • Bits 1 to 8 contain a label (label words), expressed in octal, identifying the data type.

Labels

Illustration of the airspeed indication and detection system on fly-by-wire aircraft

Label guidelines are provided as part of the ARINC 429 specification, for various equipment types. Each aircraft will contain a number of different systems, such as flight management computers, inertial reference systems, air data computers, radio altimeters, radios, and GPS sensors. For each type of equipment, a set of standard parameters is defined, which is common across all manufacturers and models. For example, any air data computer will provide the barometric altitude of the aircraft as label 204. This allows some degree of interchangeability of parts, as all air data computers behave, for the most part, in the same way. There are only a limited number of labels, though, and so label 204 may have some completely different meaning if sent by a GPS sensor, for example. Many very commonly-needed aircraft parameters, however, use the same label regardless of source. Also, as with any specification, each manufacturer has slight differences from the formal specification, such as by providing extra data above and beyond the specification, leaving out some data recommended by the specification, or other various changes.

Protection from Interference

The prohibition on the use of certain electronic devices in-flight on commercial aircraft has become a controversial issue, as, although usually justified as a safety requirement to prevent interference with aircraft avionics, it is criticized by those that argue that cell phones, WiFi, Bluetooth, and other digital radio systems are incapable of interfering with such robust protocols as ARINC.

ARINC 429 employs several physical, electrical, and protocol techniques to minimize radio and electrical interference from on-board radios and from other transmission cables.

Its cabling is a shielded 78Ω twisted-pair[1]. ARINC signalling defines a 10Vp differential between the Data A and Data B levels within the bipolar transmission (i.e. 5V on Data A and -5V on Data B would constitute a valid driving signal), and the specification defines acceptable voltage rise and fall times.

ARINC 429's data encoding uses a complementary differential bipolar return-to-zero (BPRZ) transmission waveform, further reducing EMI emissions from the cable itself.

See also

Standards

Product Manufacturers

Tutorials

References

  1. ^ a b Steve Woodward (July 11, 2002), Bill Travis (ed.), Circuit transmits ARINC 429 data (PDF), EDN Magazine
  2. ^ http://www.holtic.com/products/arinc.cfm
  3. ^ "ARINC 429 Bus Interface" (PDF). Actel. Retrieved 2009-06-24.
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